Adjustment scheme of metro air conditioning system
1、 Overview In the subway operation, the air conditioning system is a major energy consumer, among which the chiller, fan and water pump of the air conditioning system are the main power consuming equipment. To reduce the energy consumption of the air conditioning system, it can only be achieved through the correct operation of these equipment. Basically, the total energy consumption of the air conditioning system is ultimately determined by the indoor temperature and humidity environment, that is, the energy consumption of the air conditioning system maintains the difference between the temperature and humidity of the whole station and the outdoor temperature and humidity. If the indoor environment is higher than the temperature and humidity requirements that most people are satisfied with, the energy consumption of the air-conditioning system that is higher than the demand is obviously unnecessary. Therefore, in order to reduce the energy consumption of the air conditioning system, we must first analyze and study the reasonable indoor temperature and humidity environment. The ideal mode is that the supply is equal to the demand in any case. The basic principle of VAV air conditioning is to meet the requirements of the whole station personnel for different indoor temperature and humidity by changing the air volume and temperature sent into the room, and at the same time automatically adapt to the impact of the outdoor environment on the temperature and humidity in the station buildings, so as to truly meet the demand of supply. Obviously, different people have different needs for temperature and humidity, and the outdoor environment is constantly changing. In order to meet the demand of supply, the air conditioning system must be a real-time adaptive system. The subway air conditioning system is different from the ground buildings, especially the large air conditioning system. Its regulating object is the temperature of a large space, which has obvious large lag characteristics. However, one favorable factor is that the environmental control of Guangzhou Metro Line 5 adopts the screen door system, so that the controlled object is free from the interference of piston wind, which provides convenience for the control and regulation of the EMCS system, The adjustment can only consider the cooling loss at the inlet and outlet. Under normal circumstances, the heat load of subway public area mainly comes from passengers, with certain regularity. For the convenience of illustration, this section will focus on how the EMCS system realizes the regulation and control of the subway air conditioning system, focusing on the control strategies and engineering implementation methods including the regulation and control of the two-way valve at the end of the water system, the control of the supply and return water pressure of the cold station, and the control of the number of units, as described below. 2、 Air conditioning water system 1. Energy saving and optimization control of cold station 1) Energy regulation and water system control In order to ensure the normal operation of the cold source and water system, make full use of the powerful data processing and analysis functions of the EMCS system to properly regulate the system, so as to improve the operation quality, reduce the operation energy consumption and generate economic benefits. The energy consumption of the cold source and water system consists of the power consumption of the chiller host, the power consumption of chilled water, cooling water and circulating water pumps, and the power consumption of cooling tower fans. If each station at the chilled water terminal has good automatic control, on the premise that the cooling capacity of the chiller meets the demand of each station, its energy saving will be achieved by properly adjusting the operating state of the chiller, improving its refrigeration efficiency (i.e. COP value) and reducing the power consumption of the chilled water circulating pump, cooling water circulating pump and cooling tower fan. Since the cold station supplies cooling for multiple stations at the same time, the chilled water circulating pump must provide sufficient circulating water volume and meet the pressure drop of each station. The possible way to save energy is to reduce the chilled water of each station Regulating valve And make the circulating water pump run at the highest efficiency point as far as possible. In this way, the energy saving control of the cold source and water system is mainly achieved through the following three ways: maintaining the minimum cooling demand of each station, increasing the outlet water temperature of the chiller unit as much as possible to improve the COP of the chiller unit; When a two-stage pump system is used, reduce the number of chilled water booster pumps or reduce the pump speed to reduce the power consumption of the pump; Determine the number of chillers properly according to the cooling load status to improve the COP value of chillers; Under the conditions allowed by the operation of the chiller, the cooling water temperature shall be reduced as far as possible without increasing the operating power consumption of the cooling pump and cooling tower. 2) Regulation and control of chilled water At present, the secondary pump system is mostly used in the cooling circuit. When the secondary booster pump adopts variable frequency speed regulation, the operation cost is the least. The conventional operation mode is to fix the set value of the water supply temperature of the chiller (such as 7 ℃), determine the set value Δ pset of the differential pressure of the main supply and return pipes of each station at the end according to the pressure head of each station required by the design condition, and adjust the speed of the chilled water booster pump according to the relationship between the measured differential pressure at this point and Δ pset, so that the differential pressure at this point is always maintained at Δ pset. This can meet the requirements of each station, but it is not the most energy saving operation mode. If the service head of each station is required to be 50kPa under design conditions, and the pressure drop of the pipe network is 100kPa, the head of the cold water back pressure pump is 15m. Under partial load, if all stations only require 50% flow under 7 ℃ water supply temperature, the pressure drop of the pipe network is only 25 kPa. In order to maintain the terminal pressure difference of 50 kPa, the head of the booster pump should be 7.5 m. At this time, if the speed of the booster pump is reduced to 50%, its head is only 3.75m, so the speed of the pump can only be reduced to about 70%, and its working point moves to the left, deviating from the highest efficiency point of the pump. Therefore, the booster pump can only save about 50% (depending on the working curve shape of the pump) instead of "reducing the flow to half and saving the power consumption by 87.5%" as advertised by the inverter manufacturer. In fact, each station does not need a 50 kPa differential pressure at this time. If the valve is not adjusted, only 12.5 kPa differential pressure shall be required. As a result, the valve had to be turned down, and most of the pressure was consumed on the regulating valve of each station. At this time, if the water supply temperature of the chiller is properly increased and the amount of water required by each station is increased, the COP of the chiller can be increased, thus reducing the power consumption of the chiller; It can also further reduce the speed of the booster pump without maintaining the 50 kPa service head at the end, reducing the consumption of regulating valves at each station, thus further reducing the energy consumption of the pump. In fact, the requirements of each station on water volume and water temperature will not decrease at the same time. The chilled water system should meet the requirements of all stations, which depends on the EMCS system to observe the working conditions of each station, determine the maximum requirements of each station on flow and water temperature, and make appropriate adjustments. When the stations of the chilled water system use two-way valves to automatically regulate the variable water volume, the essence of the regulation is to reduce the return water temperature by increasing the water volume, thus reducing the average temperature on the water side and increasing the cooling capacity transmitted to the air side; Or reduce the water volume to increase the return water temperature, so as to increase the average temperature on the water side and reduce the cooling capacity transmitted to the air side. In this way, when the cold water valve of each station is opened to the maximum and the difference between the supply and return water temperature of each station is still large, it means that the water side of each station has insufficient pressure head, resulting in insufficient flow. The speed of the chilled water booster pump should be increased to increase the pressure head of each station, so as to increase the flow of each station; When the cold water valve of each station is opened to the maximum and the temperature difference between supply and return water is very small, it indicates that the water volume passing through each station has been large, but the water temperature is high, and the water supply temperature should be further reduced. Conversely, when the water valve of each station is closed very small and the temperature difference between supply and return water is still very small, it means that the resource head is too large and the water volume of each station is too high, so the speed of the back pressure pump should be reduced; However, when the water valve is closed very small and the difference between supply and return water temperature is too large, it indicates that each station has met the demand at a very small flow. At this time, the water supply temperature can be properly increased to increase the flow of each station. In this way, the demand of each station for water side head and water supply temperature can be judged from the valve position and water supply and return temperature difference of each station. Since the chilled water system needs to meet the requirements of all stations for water volume and water temperature at the same time, the adjustment of water temperature and water pump can be determined according to the logic in Table 3-3. The control logic of the two-stage pump system is described in Table B1-08: Table B1-08 ■ Find out the Vmax of each station with the largest valve opening and the supply and return water temperature difference Δ t1 of each station, the Vmin of each station with the smallest valve opening and the supply and return water temperature difference Δ t2 of each station; ■ If 80% ≤ Vmax ≤ 90%, the set values of water temperature of water pump and chiller shall be maintained; ■ If Vmax ＞ 90%, Δ t1 ＞ Δ tmax, the flow is insufficient, and the pump speed should be increased by 5%; ■ If Vmax ＞ 90%, Δ t1 ＜ Δ tmin, and t supply ＞ t supply min, the water temperature is too high, and the set value of the outlet temperature of the chiller should be reduced by 0.25 ℃; ■ If Vmax ＜ 80%, Δ t2 ＞ Δ tmax, and t supply ＜ t supply, max, the water temperature is too low, and the outlet temperature of the chiller should be increased by 0.25 ℃; ■ If Vmax<80%, Δ t2>Δ tmin, the flow is too large, and the pump speed should be reduced by 5%. Where Δ tmax and Δ tmin are the expected maximum and minimum temperature differences of supply and return water respectively. When the designed temperature difference between supply and return water is 5 ℃, Δ tmax=6 ℃ and Δ tmin=4 ℃ can be taken. If the allowable temperature difference is too large, the required flow can be reduced, but the outlet temperature setting value of the chiller should be reduced accordingly to reduce the efficiency of the chiller. If the allowable temperature difference is too small, although the water temperature setting value of the chiller can be properly increased, the pump flow will increase and the power consumption will increase. The above regulation methods can maximize the operation efficiency of the chiller and reduce the power consumption of the pump under the premise of meeting the operating conditions of each station, so as to achieve the maximum energy-saving effect. At the same time, this adjustment mode also has good stability. For example, when Vmax is greater than 90% and Δ t1>Δ tmax, according to the above logic, the pump speed should be increased. As a result, the flow of each station increases, the temperature at the air side decreases, and each control valve is gradually closed accordingly until the valve position with the largest opening falls below 90%, and the regulation of the water pump stops. According to the traditional method of maintaining the terminal pressure difference, when each station requires to reduce the flow and turn down the valve, the terminal pressure increases, which reduces the pump speed. This will lead to the flow of each station being smaller, and the temperature of the air side gradually rising. Then, the valves are opened in succession to increase the flow, which causes the pressure of the terminal pressure monitoring point to decrease, and then causes the pump speed to increase. Since each station adjusts its valves according to the working conditions, which has a large thermal inertia and time delay, and the end pressure change inertia caused by the adjustment of valves and pumps is very small, it is easy to cause the above oscillation process, so it is necessary to carefully design the control algorithm and set the adjustment parameters to eliminate this oscillation. In contrast, the regulation mode in Table B1-11 is a self stabilizing regulation process in mechanism, which is recommended. The condition of the above regulation method is that the air conditioning of each station is regulated by two-way valve variable flow and controlled by a controller. The field controller of each station shall have the communication function with the controller of the cold station. The actual demand of each cold water station is obtained through communication, so as to realize the regulation that just meets the requirements of each station. If the current engineering situation of Guangzhou Metro Line 4 does not meet the above adjustment conditions, we have studied a set of optimization scheme for differential pressure method adjustment, which has been very successful in the practical application of previous projects. The specific description of this scheme is shown in Figure B1-14 below:
Figure B1-14 In the control logic in Table B1-08 of the above regulation method, it is not difficult to find the changes in the temperature difference between the supply and return pipes and the opening of the valves. The target is the cooling capacity demand of the room, which is derived from the water temperature at the outlet of the chiller and the pressure difference between the supply and return water, that is, the demand for the room cooling capacity affects the opening of the valves. When the valves are opened to the maximum extent, It will affect the increase of the temperature difference of the supply and return pipes. When the temperature difference changes to the limit, and the room cooling demand cannot be met, it is necessary to adjust the booster of the secondary pump. When the speed of the secondary pump reaches the limit (the limit refers to the range of the best efficiency of equipment operation, such as the highest operating efficiency when the speed is 80%~90%), The water temperature at the outlet of the chiller can only be reduced to meet the requirements. Let's analyze again below, when the room cooling demand is certain, the relationship between the four variables of water temperature t at the outlet of the refrigerator (this parameter is set as a fixed value, and the set point of this fixed value is the middle value of the cooling efficiency range, which can be reduced only when all parameter changes cannot meet the load requirements), supply and return water pressure difference △ P, supply and return water temperature difference △ t, and air conditioning two-way valve position L, See Table B1-09 below: Table B1-09 ■ 1. When t and △ t are fixed, △ P ∝ L; ■ 2. When t and L are fixed, △ P ∝ △ t; ■ 3. When t and △ P are fixed, L ∝ △ t; ■ 4. When changing t directly affects △ P, it indirectly affects L and △ t; After clarifying the relationship between the above parameters, we can easily draw the following conclusions - the logical relationship between the parameters in Table B1-10 (because the indirect influencing factors are lagging, this logical relationship can be organized according to each link, ignoring the indirect influencing factors): Note to Table B1-10: ■ V0: speed of forced draft fan T0: set value of air supply temperature △ t0: set floating value of air supply temperature ■ L: position of two-way valve △ P: set value of differential pressure △ △ P: set floating value of differential pressure ■ V1: rotational speed of secondary pump t: water temperature at outlet of cooler
See more comments
